Currently and in the past, there has been the need to electrically test semiconductors in a sub-zero environment. With military, aerospace and commercial test specifications becoming more stringent, the need for this type of testing is growing rapidly.
In the past, semiconductor devices have been tested for their operation in sub-zero environments by first placing the device in an environmental chamber and cooling the device to -55.degree. C. for sometimes as long as ten days. The devices were then taken out of the chamber and tested at room temperature to ascertain the effects of the previous exposure to low temperatures. No attempt was made to test the function of the device in the chamber while the device mass was stabilized at -55.degree. C.
Newer systems have attempted to test the operation of semiconductor devices while at sub-zero temperatures, but these systems are configured for only one type and size of device and have, in the main, resulted in freeze-ups and erratic results as will be subsequently described.
One of the important problems with the prior art systems is the freeze-up of the so-called "test head," which in general, is a unit which houses the drive circuitry for the functional testing of a particular device. These units typically have ten to twenty integrated circuits mounted to printed circuit boards within the test head. At the top of the unit is a printed circuit load board, usually circular in configuration, which transmits the test signals to a central socket into which the device under test (DUT) is plugged. As will be described, freeze-up of the circuits in the test head is a problem that affects the results of the functional test being performed.
In the first of the automated units to be described, the test head is insulated from the test site, but leakage causes test head malfunction and freeze-up nonetheless. In the second of the systems to be described, no attempt is made to insulate the test head circuits from sub-zero gases used to cool the device under test. Thus, in both of the systems to be described, test head freeze-up is a significant problem.
For instance, in completely automatic systems such as the Automatic Device Handler manufactured by Trigon Industries of Mountain View, Calif., several input slides are utilized. Means are provided for moving the individual devices from the input slides to a test site. Gangs of contact pins at the test site are then actuated to contact the device, and the device is then tested. After testing, the device is channeled to one of two output slides, depending on whether or not the device meets the test criteria. In this type of equipment, the device is initially cooled or presoaked after it is loaded into the input slide, and it is further cooled at the test site to bring it to the desired temperature. The reason for the subsequent cooling at the test site is that there are temperature variations that occur during handling. These temperature changes are primarily responsible for condensate and subsequent freezing. After test, the device is allowed to return to ambient as it is removed from the test site and exits down an output slide.
What will be appreciated in this automatic loading and unloading system is that the slide configurations as well as the test site configuration must be changed to accommodate different sizes of semiconductor packages or components, which is an extremely costly endeavor. Also, one handler can accommodate only one type of device, currently either a dual-in-line package (DIP) or a leadless chip carrier (LCC). At the present time, automatic handlers are not available for flat packs, transistor outline (TO) packages, axial lead components or pin grid arrays.
Thus, for certain packages, there is no possibility of a retrofit because the package configurations preclude conventional automated contacting and device handling. Even if such fixturing exists, the only way to test a given type of device is to purchase a testing unit dedicated to a particular type of device and then to buy retrofit kits for each size.
In addition to the difficulty in retrofitting such a machine for different sizes of devices, it is with extreme difficulty that the temperature in the presoak section can be made to match the test site temperature, with the result that condensate occurs which causes freeze-ups and jamming. Also, even if the test site is calibrated with a time-temperature profile for a given semiconductor device, the actual temperature of the device under test is not directly measured. Rather, it is the refrigerated gas stream temperature which is measured, and this leads to inaccurate results. Moreover, cold air often leaks from the internal test site to the external test head which causes moisture in the air surrounding the external test head to freeze. This, in turn, freezes up the external test head even though the test head is insulated from the test site.
In addition to the test head freeze-up problem with such automated equipment, there is also the problem that the mechanical sorting apparatus heats up the devices to be tested, either due to the internal friction during the handling process or due to the amounts of energy delivered to the solonoids utilized in moving the devices from one position to another. This energy dissipates to the test environment and alters the test environment in both an uncontrolled and unsensed manner.
Thus, if the testing of any given device is to be at -55.degree. C., while it is possible to initially cool down all of the devices in the slides to -55.degree. C. in a presoak cycle, the temperature of their mass may be altered when they are moved to the test site which is invariably at a different temperature. This leaves devices at an unestablished temperature at the test site. In order to test such devices, they must be brought back to a predetermined test temperature at the test site, which requires the calculating of another time-temperature soak profile.
Moreover, unless extraordinary measures are utilized to remove all moisture, freeze-ups occur inside the unit at the test site which can result in the jamming of all moving parts. At this point, the test site must be opened and exposed to the ambient until all parts thaw. Unfortunately, moisture carried in room air is deposited on the test site, and unless the test site is thoroughly dried, it will freeze up again. It will be noted that the test site, along with its actuating apparatus and its delicate, closely-packed lead frame structure, is within the environmental chamber which subjects all test site leads and movable contacts to the possibility of freeze-up, condensation-caused shorting or unwanted resistance and capacitance between leads.
Having described in some detail certain types of automatic device handlers which are dedicated devices utilizing slides and manipulators, and in an effort to provide a device which will work with all types of test heads, temperature-forcing systems are currently used in which the device, test socket and test head are encapsulated in a chamber which comes down over the entire test head assembly. A robot arm, along with a cylindrical chamber opened at its base, comes down over the entire test head and is sealed at its base to the socket by a rubber pad which overlies the test head and has a hole in it through which the socket projects. The individual device is then cooled by the introduction of cooled dry gas into the chamber which floods the entire test head.
The first problem with respect to this type of testing is that it often takes three to five minutes to cool down the device under test, which is a prohibitively long period of time. Moreover, the cool-down period is extremely critical since, in these devices, there is no temperature sensing of the mass of the device. Rather, a time-temperature profile is utilized which assumes that the mass is at the correct temperature when a certain period of time has elapsed. Not only may this assumption be inaccurate, operators of such machines are often unable to effectuate proper testing due to the boredom associated with waiting minutes for the device to cool down. As a result, the length of cool-down time is either underrun or overrun due to operator error.
The second problem is that the entire test head and device under test are exposed to room air prior to the robot arm moving the cylindrical chamber into place. This captures the moisture in the air which condenses and causes test head freeze-up.
Thirdly, on cool-down, the whole test head is flooded with sub-zero gas, despite the rubber sealing normally used. This exposes all electronics in the test head to freezing gas which can cause electrical malfunction.
Thus, with respect to these thermal-forcing systems, since the entire test head is initially exposed to ambient conditions, moisture in the air is captured as the cylindrical chamber comes down. The result is significant freeze-up of all parts so that the testing throughput is severely limited. Additionally, the rubber pad on which the cylindrical test chamber comes to rest has to be changed as frequently as the DUT configuration is changed, such that the expense for the utilization of this robot-actuated encapsulation chamber increases dramatically. Further, the entire procedure results in the build-up of electrostatic charge which precludes accurate test results.
More specifically, every time the cooled cylinder is removed, all parts of the test head are exposed to room air which is moisture-laden. This causes condensate to form on the test head and, after a few tests, the condensate freezes up. If this were not enough of a problem, there are problems in temperature control because temperature-forcing systems are based on the use of cold forced air applied for a highly critical period of time. To determine the soak time, calculations are made which are supposed to ensure that the device under test is at a specific temperature. Since no temperature sensors are located on the device under test, the above procedure is only approximate, and even if the calculations correspond to what is actually occurring, soak time is still critical and can vary depending on the amount of condensate that builds up between tests. To correct this, the test head must be allowed to thaw to room temperature every few devices. This results in the deposit of even more condensation from the air, which occurs when the forcing system is removed from the test head for a long period of time, causing even greater freeze-ups.